Thanks for including all link elements as tab stops and handling space key on them. This really fixes the issue of tabbing gets stuck in Extensions Manager dialog, but it also creates new issues that we may have to solve later. The issues that I'm seeing are 
  - 'X' button in Switch Language dialog is in the tab loop.
  - The two tabs in Extension Managers are in the tab loop, which is both good and bad. Good in that users can use keyboard to switch tab, but bad in that the focus is reset to search field after switching. But switching tab with keyboard is totally another area of keyboard accessibility and we don't need to bother with this pull request.